The present invention relates to the field of optical communications, and in particular to the field of beam steering for free space optical transceivers. More particularly, the present invention relates to a beam pointing and tracking apparatus and method carried out internally within the respective telescopes of a transmitter and receiver, without the use of external gimbals.
Laser communications systems are today employed in a vast array of applications, including without limitation communication with aircraft and satellites from ground positions. A laser communication system generally consists of a transmitting terminal and a receiving terminal. A transmitting terminal typically receives an electrical signal from a signal source, and converts the electrical signal into an optical signal. The transmitting terminal then transmits the resulting optical signal using a transmitting telescope. The receiving terminal receives the optical signal through a receiving telescope, which focuses the optical signal into an optical photodetector, and then converts the optical signal back into an electrical signal.
In order for a receiving terminal to receive an optical signal from a transmitting terminal, the terminal telescopes must be properly aligned. This alignment process is known as beam steering. Generally, beam steering may be defined as changing the direction of the main lobe of a radiation pattern. In optical systems, beam steering is the manipulation of a transmitting telescope or receiving telescope to point in a desired direction. Other applications for beam steering, in addition to optical communications, include laser illumination, laser designation, laser radar, pointing and tracking, and remote optical sensing.
Beam steering in optical systems may be accomplished by changing the refractive index of the medium through which the beam is transmitted, or by the use of mirrors or lenses. In particular, some existing non-gimballed beam-steering solutions include acousto-optics, liquid crystals, electro-optics, micro-optics, galvanometer or magnetic mirrors, and micro-mirror arrays. These types of systems, however, have generally proven to be unwieldy, or lack the speed, precision, and reliability necessary for high-speed, long-distance free-space optical communications. Thus the most common means for beam steering in optical communications systems is by the use of a motorized gimballing system. A gimbal is a mechanical apparatus to allow a suspended object to rotate freely along two simultaneous axes, within a defined angle of view. Gimbals are well known in the art, having been used, for example, since at least as early as the sixteenth century in the suspension of maritime compasses. A gimballing system used for the alignment of an optical transmitter or receiver typically moves the entire transmitting or receiving telescope through the required field of view. Often, the transmitter and receiver telescopes are mechanically coupled so that the transmitted beam is in the exact direction of an incoming optical beam for collection by the receiving telescope, the two telescopes thereby operating with a common gimballing system.
Accurate alignment of the transceiver system is essential for free space laser communications systems. Therefore, gimballing systems must provide accurate alignment angular resolution in order for the receiver telescope to efficiently collect the incoming optical beam. Conversely, the transmitter telescope must be able to accurately point its beam so that a remote-receiving terminal can efficiently collect the optical signal for the photodetector. Mechanical gimballing systems have been favored in many free-space optical communications systems because they can provide very fast alignment times coupled with high angular resolution.
Gimballed beam-steering systems do, however, suffer from several important disadvantages. Such systems are quite heavy due to the weight of the mechanical components, motors, and servos necessary for such a system. While weight may not be as important a factor in the design of a land-based system, weight is of paramount importance in aircraft and satellite design. Gimballing systems are also quite bulky due to the required mechanical components, which is also a significant disadvantage in the design of airborne and spaceborne systems. Finally, mechanical gimballing systems require the use of a great deal of electrical power, far more power than is typically consumed by the electronics associated with an optical receiver or transmitter system. Again, while power consumption may not be as important a factor in permanent ground-based systems, it is a critically important factor in airborne and spaceborne systems, as well as in mobile ground-based systems such as may be mounted on land vehicles.
What is desired then is a beam pointing and tracking system that provides high speed and high angular resolution, with reduced size, weight, and power consumption as compared to traditional gimballing systems now employed in laser communications terminals.